Sponge iron, metallized pellets or reduced metal materials are produced by the direct reduction of ores. "Metallized" in this sense does not mean coated with metal, but means nearly completely reduced to metal, i.e., always in excess of 75% metal, and normally in excess of 85% metal in the product. This metallized product is suitable for charging directly to a metal refining furnace as the feed material. In ferrous metallurgy, the product referred to is metallized iron material, which is charged directly to a steelmaking furnace, such as an electric arc furnace. Steel plants which utilize metallized iron as a feed material have no need for metallurgical coal or coke. Further, such plants are economical at small capacities and thus do not require the high capital investment of plants which employ blast furnace.
One of the problems associated with the use of sponge iron as a raw material in steelmaking is its inherent tendency to reoxidize upon exposure to atmospheric conditions. Hot sponge iron is extremely reactive and oxidizes spontaneously if contacted by oxygen in any form. Thus, sponge iron must be cooled in a reducing or neutral atmosphere. At room temperature, sponge iron is so reactive that it oxidizes even when stored in the open air. Contact with water, likewise, causes rapid oxidation, which is commonly termed rusting. Since the oxidation of sponge iron is an exothermic reaction, this oxidation can result in spontaneous heating and ignition of the sponge iron during storage or transport. For this reason, metallized sponge iron has been classified a hazardous material by the U.S. Coast Guard, and its bulk shipment in the unstabilized condition is prohibited.
In some instances, reduced iron in such form as sponge iron or metallized iron pellets is produced in an integrated steel plant as a raw material for the steelmaking furnaces. If it were possible to feed the hot reduced iron, at a temperature above 500.degree. C (about 930.degree. F), directly into the steelmaking furnace, this would result in a more economical steelmaking process, inasmuch as the energy requirements would be greatly reduced and higher productivity would be obtained. It would be imperative that hot sponge iron material be transported and handled in a controlled atmosphere, as exposure to atmospheric air would result in an extremely hazardous situation. If the hot sponge iron could be passivated sufficiently that it could be transported by conventional equipment with a minimal heat loss, the steelmaking process for which it serves as a raw material could realize the full benefit of its heat content with attendant savings in energy consumption.
Passivation of sponge iron is also desirable because oxidation of sponge iron, after having once been reduced, requires a second reduction with an attendant increase in energy consumption and cost.
Many attempts have been made in the past to overcome, or reduce, the reoxidation of metallized pellets and to passivate sponge iron. Illustrative examples include the proposal to cover a bulk shipment of sponge iron with a thin polyurethane foam coating or other type of plastic film to prevent oxygen or moisture from contacting the sponge iron. It has also been suggested to cover such a bulk shipment with a thin glass coating. U.S. Pat. No. 3,125,437 teaches a process for passivating sponge iron against oxidation in air by creating a thin protective skin on the sponge iron surface. Hot briquetting with roll type briquetting machinery are taught in U.S. Pat. Nos. 3,116,996 and 3,174,846 to densify the sponge iron, thereby minimizing the surface area of the reduced iron ore exposed to the oxidizing elements. These illustrative, but not exhaustive examples demonstrate the many attempts to solve the problem.
Coatings on sponge iron require the use of a foreign material which contaminates sponge iron without guaranteeing passivation. Such coatings are easily damaged, for instance, a mere shifting of the material in its container during transit may rupture the coating. Although a protective oxide skin is a proven inhibitor to oxidation in air, it is subject to rusting to hydrated ferric oxide. Thus, such skin does not prevent further oxidation by rusting.
Heretofore, the hot briquetting of sponge iron has been a very promising process for passivation inasmuch as it can be used to passivate bulk shipments to a high degree, as well as to passivate hot sponge iron with temperatures as high as 900.degree. C (about 1650.degree. F), so that it can be transported on conventional hot conveying systems at high temperature without either a prohibitively high loss of metallization or spontaneous ignition. Densification of sponge iron, at least on its surface, is accomplished by hot briquetting. The exterior of the briquet is compressed to a dense layer which is stable or passivated. The interior of the briquet remains less dense, i.e., spongy, and thus is active and readily oxidized, but is protected by the more dense surface layer.
Hot briquetting encounters certain mechanical problems. Before the briquetting rolls start to wear, single briquets are easily produced. As soon as wear begins, briquets become connected to each other by webs, which requires that they be broken apart prior to shipment or handling. As roll wear increases, the problem of breaking the briquets apart becomes more and more difficult. In addition, the breaking procedure produces fines and exposes the less densified interior of each briquet to oxidation, particularly if the breakage occurs through the briquet rather than through the web. With increasing web thickness due to increasing wear, this occurs more and more frequently. Thus, although the greater proportion of each briquet is passivated, there is still a sufficient proportion of the briquet which is less passivated and subject to reoxidation with a high loss of metallization.
For known strip breaking mechanisms, see German Pat. No. 1,533,827.